There exist many techniques for heating food products electrically, employing alternating current. U.S. Pat. No. 4,417,132 describes an example of such a technique applied to liquid foodstuffs. U.S. Pat. No. 3,996,385 discloses alternating current electrical heating of potatoes immersed in an electrolyte solution, wherein the current level is varied along the processing path. U.S. Pat. No. 3,632,962 illustrates the application of alternating current to meat and similar food for heating thereof by direct contact with electrodes.
U.S. Pat. No. 3,651,753 describes a control circuit for alternating current cooking appratus which compensates automatically for changes in load resistance and/or supply voltage.
All of the above patents indicate the use of alternating current at mains frequencies, i.e. 60 Hz or below. It has been found by applicant that the use of mains frequencies is unsuitable for food processing due to the resulting electrolysis and damage to the structure of the food products. The electrolysis may cause chemical contamination of food products through oxidation and/or reduction. For example, ordinary cooking salt may be broken down into hydrogen, chlorine and sodium hydroxide. An additional difficulty with the prior art apparatus is the tendency of the electrodes to dissolve, possibly resulting in contamination of the food products.
It has been found by applicant that application of AC current at mains frequencies also causes substantial breakdown of the cellular structure of the food products, which is often undesireable.
Although the use of high frequency radiation is well known in microwave cooking applications, the use of such high frequencies has not been taught or suggested in the prior art for electroheating applications wherein AC current is caused to pass through a food product.
Induction heating of food products is also well known in the art and is described in the following U.S. Pat. Nos. 4,265,922, 4,241,250 and 3,498,209. In all of these patents, eddy currents are induced in a metal housing or enclosure which is heated and the heat is transferred to the food by conduction.